Hi all,
I've been scratching my head for a few days regarding the final details of a constant current sink using a small signal pentode.
My idea is to use a TL431 to set the g1 bias point. This seems like a good enough voltage reference with good temperature stability and tight control on the voltage and low noise.
I was thinking of stacking on top of this a string of 5x36V zeners to set the screen voltage. I was thinking of using a single stack for both channels of a preamp. It seems to me that the noise produced by the zener string will not be amplified and it is very low compared to the line level signal it will be working with.

Space is at a premium and this has led me to this idea. Am I foolish to go down this path, or am I right in assuming the zener string will have minimal effect.

The project is a super linear cathode follower with a pentode input configured as an ultralinear stage.
Its mad and over complex but it uses all the parts of two ECF80's which I have. It seems obvious to just put the TL421 as a CCSink, but I want to try this out and see if it sings.
The Supertex Mosfets are not easily available and would require an order to America which I don't want to do.

I've been mulling this circuit over for about a year now, and it isn't easy to get the balance right.

I use a 6AU6 as a sink in the tail of the input 6SL7 LTP stage of my PP amp. No zeners are needed, just a negative rail of 120v. The pentode's current is determined by its cathode resistor and screen voltage.

Actually, it's a good idea to bias up the control grid so you can use a higher value cathode resistor to give a higher impedance, but you can do this easily enough with resistors acting as a voltage divider between ground and the negative supply. Again, no need for zeners. I suggest you have a look at Morgan Jones's Valve Amplifiers if you have it. If not, buy it.